How to reset MySQL root password

[root@tse2 tmp]# kill -9 9840 1
[root@tse2 tmp]# ps -ef |grep mysql 

[root@tse2 bin]# mysqld_safe --defaults-file=/etc/my.cnf --skip-grant-tables &
[1] 4854 

(product)root@localhost:mysql.sock [(none)]> use mysql;
Database changed
(product)root@localhost:mysql.sock [mysql]> update user set authentication_string=password('123456') where user='root';
Query OK, 0 rows affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 1

(product)root@localhost:mysql.sock [mysql]> flush privileges;
Query OK, 0 rows affected (0.00 sec)

[root@tse2 bin]# mysqld_safe --deaults-file=/etc/my.cnf &
[2] 6720
[root@tse2 bin]# 2020-01-16T02:55:45.223195Z mysqld_safe Logging to '/mysql/mysql3306/logs/error.log'.
2020-01-16T02:55:45.262302Z mysqld_safe A mysqld process already exists

[2]+ Exit 1 mysqld_safe --deaults-file=/etc/my.cnf
[root@tse2 bin]#mysql
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

[root@tse2 bin]# mysql -uroot -p
Enter password: 
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 4
Server version: 5.7.23-log MySQL Community Server (GPL)

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(product)root@localhost:mysql.sock [(none)]> use mysql
Database changed
(product)root@localhost:mysql.sock [mysql]> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
|mysql |
| performance_schema |
|sys|
+--------------------+
4 rows in set (0.01 sec)